Form 7 is already a mix of all the previous styles, so mixing it with the others is pointless.
your mixing form VI with VII
for those who don't know, these are the forms:
Form I: Basic
Form II: very powerful. mostly skilled manoeuvring, dueling, and devastating out flanking of an opponent(could be wrong with this one, but oh well) Count Dooku uses this
Form III: the defensive form. great when surrounded by enemies, works great with moving meditation, which heightens reflexes and awareness, as well sensing danger. Obi-Wan Kenobi's true style
Form IV: the acrobatic form. uses lots of jumping and outmanoeuvring. has a weakness to blaster fire. note: both Qui-Gon Jinn and Yoda are seen using this form, but both are so strong that blasters are nothing to them, even with this form.
Form V: the stand and fight form. uses strong swings and deflecting blasts. great for cutting through droids, clones, stormtroopers, rebels, or even Sith and Jedi, as Anakin well demonstrates.
Form VI: the mix. a mash-heap of forms 2,3,4, and 5. also known as the Counsellor's form, as it, besides Form I, is the easiest to learn. it is also the form used when wanting to use 2 lightsabers. while not conformed(unless i'm mistaken), it is assumed that Ahsoka Tano uses this in the Clone Wars
Form VII: the dark form. the one that requires you to tape into the dark side, but is also best against the dark side. use harsh, strength filled attacks. also has some weakness to blaster fire. it is best to master another form to fall back on if Vaapad(Form VII) proves too much in battle. 2 jedi are known to use this, most notably, Mace Windu.(note, this form, in addition to Palpatine's plan, was what kept Mace alive so long)